Load-Carrying Capacity of Damaged Concrete Structures

Licentiate thesis, 2008

Abstract A growing need for better assessment of existing structures has revealed the need for improved understanding of structural effects of deterioration. The two most common causes of deterioration in concrete structures are corrosion of reinforcement and freezing of concrete. Previous research has been concerned primarily with the causes and mechanisms of corrosion and frost deterioration; relatively little attention has been given to the important practical problem of assessing the residual load-carrying capacity of damaged structures. This study offers a methodology to analyze the mechanical behaviour of reinforced concrete structures damaged by frost or the corrosion of reinforcement. It is proposed that the effect of damage be modelled by reducing material properties, such as compressive strength, tensile strength, elastic modulus, as well as by modifying the bond properties and geometry. For frost-damaged concrete, the effect of freezing on material properties was gathered from the literature. It was proposed that the effects of internal frost damage and surface scaling can be modelled as changes of material properties and geometry, respectively. The methodology was used in analyses of concrete beams affected by internal frost damage, by using non-linear finite element analysis at component and structural levels. Comparing the results with available experimental data indicated that the change in failure mode and the decrease of load-carrying capacity due to frost damage can be predicted by using the proposed methodology. For corroded structures, the decrease in ductility and area of the corroded reinforcement and the behaviour of cracked concrete around corroded reinforcement were extracted from previous publications. The one-dimensional bond-slip model given in the CEB-FIP Model Code 1990 was extended to include corroded reinforcement. Furthermore, a method to calculate the anchorage length from the bond-slip relation was developed. The proposed methodology was used in analyses of beams, using both non-linear finite element analyses and analytical methods. Comparisons with results, taken from the literature, showed that the methodology could reasonably well estimate the load-carrying capacity and failure mode of corroded beams. The thesis provides a guideline, for the assessment of concrete structures damaged by freezing or corrosion, which can be applied in engineering practice. The change of material properties for the types of damage is quantitatively described. However, some uncertainties exist: the main ones for frost-damaged concrete are the elastic modulus and bond properties; for corroded bars they are the ductility and the bond properties when the cover has spalled off.